How to find good people in a field?

So I'm a 3rd year student, not quite at the stage to be applying for grad school, but almost there. I'm trying to get a preliminary idea about where I'd like to go/apply to. From what I've gathered, of course it's great to go to a "top ranked" school (according to the internet rankings anyway), but it's better to go to a school that is good in your intended area, as if it's a "good" school, but no one there does what you wanna study, there's a good chance of not being admitted since you'd essentially be on your own. So I'd like to know of a process to find out who's good in a particular area. I'm going to be applying for pure math, but just generally speaking, how does one determine "it'd be better to go to X for algebra since they person Y, or don't go to school Z because although they have people in algebra, they don't publish much" or something along those lines. All I have to work with right now are things like US-news rankings and what not, but I somehow feel like those aren't complete trustworthy all the time. Any advice? Sorry if I come off misinformed - I very well might be. But it just seems like a lot of forums, people, etc. just talk about the "top ranked" schools, and I'd like a different perspective.

I was recently in a situation similar to yours and found a solution quite by accident. If your university or one near you hosts a conference, go to it.

Even though I didn't understand anything at the conference I went to it showed me who's doing what, who has their name on everything, and, as a side note, what's trendy in the field. I have a very limited experience, but it seems like the people that go to conferences are the people that are doing important stuff -right now-, and they're from all over the place.

I may be naive, but at least it'll show you whether you have what it takes to sit through 8 hours a day of bleeding edge math, and give you a starting point of where to look in the literature for people that get cited a ton.

Now you might have me there, I haven't experienced any exclusion or fees, myself. If you really wanted to get into that one you could probably email the organizers. Provided their contact info is posted somewhere.

So I'm a 3rd year student, not quite at the stage to be applying for grad school, but almost there. I'm trying to get a preliminary idea about where I'd like to go/apply to. From what I've gathered, of course it's great to go to a "top ranked" school (according to the internet rankings anyway), but it's better to go to a school that is good in your intended area, as if it's a "good" school, but no one there does what you wanna study, there's a good chance of not being admitted since you'd essentially be on your own. So I'd like to know of a process to find out who's good in a particular area. I'm going to be applying for pure math, but just generally speaking, how does one determine "it'd be better to go to X for algebra since they person Y, or don't go to school Z because although they have people in algebra, they don't publish much" or something along those lines. All I have to work with right now are things like US-news rankings and what not, but I somehow feel like those aren't complete trustworthy all the time. Any advice? Sorry if I come off misinformed - I very well might be. But it just seems like a lot of forums, people, etc. just talk about the "top ranked" schools, and I'd like a different perspective.

Ignore internet rankings. Instead, read journals in the field of interest, and see who is publishing articles in the area of interest.

If one is applying for pure math, then read journals in math, particularly from American Mathematical Societyhttp://www.ams.org/journals

I don't know about mathematics conferences - in physics, you usually have to pay (some 3-digit amount) to attend a conference.

Not usually. For example, an undergraduate student who is a member of the SPS/APS often gets a substantial discount, and some time even free, to attend certain conferences and workshop. Some of the APS conferences often have funds to cover undergraduate travel. I know for sure that the Particle Accelerator Conferences have 100% travel/registration funds (limited number of them) to support undergraduate/graduate students to attend such conferences.

To me, the best ways to find out who's who in the field is to keep an eye on journals, and, most obviously, to ask your faculty members. Talk to the professor who is either in the same field, or in a closely related field, and ask him/her to recommend a few papers or review articles in that area. Once you have that as a starting point, all you need to do is follow the "paper trail", which is all the citations in those papers (review articles are the best) and figure out who's doing what where.

Staff: Mentor

Not usually. For example, an undergraduate student who is a member of the SPS/APS often gets a substantial discount, and some time even free, to attend certain conferences and workshop. Some of the APS conferences often have funds to cover undergraduate travel. I know for sure that the Particle Accelerator Conferences have 100% travel/registration funds (limited number of them) to support undergraduate/graduate students to attend such conferences.

Well, most participants are within some research group and don't have to pay it personally anyway.
I have no idea about US-specific conferences, but the relevant international conferences in my field all have significant fees, sometimes with support programs. To make it worse, those programs can be limited to participants with own talks/posters.
An example from ICHEP:

A limited amount of grants will be awarded to support PhD students and postdocs who otherwise would have problems to attend the conference.

This support will cover the registration fee and accommodation. Travel expenses, either partial or full, could also be covered, based on individual requests and motivation.